Phospholipid, triglyceride, cholesterol, and cholesteryl ester contents were measured in unaffected and atherosclerotic areas of human aorta and in a suspension of enzyme-isolated cells from these segments. Aortic tissue and the cells isolated from it, as well as intimal and medial cells, significantly differ in lipid content. As lipoidosis develops in an atherosclerotic lesion, lipids accumulate unevenly in the tissue and cells. In zones of fatty infiltration, lipids accumulate, apparently, mainly inside cells while in the fatty streak and atherosclerotic plaque they predominate in the extracellular space. In a suspension of cells derived from both an atherosclerotic lesion and the underlying media, cholesteryl esters are the main component of excessive fat. In the primary culture of cells enzyme-isolated from unaffected intima, fatty streak, and plaque, the lipid content and composition are retained until Days 12 to 14 and are similar to those of freshly isolated cells.